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EQUITY-BASED COMPENSATION - Performance share units (Details) - Performance share units
$ in Millions
3 Months Ended 6 Months Ended 12 Months Ended
Jun. 30, 2019
USD ($)
Jun. 30, 2018
USD ($)
Jun. 30, 2019
USD ($)
item
Jun. 30, 2018
USD ($)
Dec. 31, 2018
USD ($)
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of times shares issued for each performance share settlement | item     1    
Grant date fair value of PSUs     $ 6.9   $ 5.9
Compensation expense $ 0.8 $ 0.4 1.7 $ 0.6  
Fair value of outstanding shares     11.8    
Unrecognized compensation expense $ 8.6   $ 8.6    
Weighted-average remaining life     1 year 9 months 18 days    
Minimum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of times shares issued for each performance share settlement | item     0    
Maximum          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Number of times shares issued for each performance share settlement | item     1.75    
Return on assets Less than 9.6%          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of Target PSUs Earned     0.00%    
Return on assets 9.6%          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of Target PSUs Earned     50.00%    
Return on assets 12%          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of Target PSUs Earned     100.00%    
Return on assets 14.4%          
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]          
Percentage of Target PSUs Earned     175.00%